KIEV (REUTERS) - Ukrainian President Petro Poroshenko said on Saturday Russian opposition politician Boris Nemtsov was murdered because he planned to disclose evidence of Russia's involvement in Ukraine's separatist conflict. Poroshenko paid tribute to Nemtsov, who was shot dead late on Friday, and said the fierce critic of President Vladimir Putin had told him a couple of weeks ago that he had proof of Russia’s role in the Ukraine crisis and would reveal it. “He said he would reveal persuasive evidence of the involvement of Russian armed forces in Ukraine. Someone was very afraid of this... They killed him,” Poroshenko said in televised comments during a visit to the city of Vinnytsia. More than 5,600 people have been killed since pro-Russian separatists rebelled in east Ukraine last April, after the ousting of a Moscow-backed president in Kiev and Russia's annexation of ...